The present invention relates generally to methods and apparatus for treating disorders by cranial nerve stimulation. More particularly, it concerns methods and apparatus for treating vocal cord disorders by vagus nerve stimulation, particularly stimulation of one or more branches of the vagus nerve enervating the vocal cords.
There have been many improvements over the last several decades in medical treatments for disorders of the nervous system, such as epilepsy and other motor disorders, and abnormal neural discharge disorders. One of the more recently available treatments involves the application of an electrical signal to reduce various symptoms or effects caused by such neural disorders. For example, electrical signals have been successfully applied at strategic locations in the human body to provide various benefits, including reducing occurrences of seizures and/or improving or ameliorating other conditions. A particular example of such a treatment regimen involves applying an electrical signal to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. No. 4,702,254 to Jacob Zabara, which is hereby incorporated in its entirety herein by reference in this specification. Electrical stimulation of the vagus nerve (hereinafter referred to as vagus nerve stimulation therapy or VNS) may be provided by implanting an electrical device underneath the skin of a patient and performing an electrical stimulation process, which may optionally include a sensor to detect a symptom of a disorder or condition of interest, which may then be used to trigger the electrical stimulation. Alternatively, the system may operate without a detection system once the patient has been diagnosed with a disorder, and may periodically apply a series of electrical pulses to the vagus (or other cranial) nerve intermittently throughout the day, or over another predetermined time interval.
A nerve bundle to which neurostimulation therapy is applied may comprise up to 100,000 or more individual nerve fibers of different types, including larger diameter A and B fibers which comprise a myelin sheath and C fibers which have a much smaller diameter and are unmyelinated. Different types of nerve fibers respond differently to different types of stimulation signals. These different responses among nerve fiber types reflect, among other things, their different sizes, conduction velocities, stimulation thresholds, and myelination status (i.e., myelinated or unmyelinated). Therefore, the patient's body may respond differently depending on which type(s) of nerve fibers are the target of the stimulation therapy. In general, the larger, myelinated A and B fibers have a lower stimulation threshold than the unmyelinated, smaller C fibers.
The vocal cords are a pair of bands of elastic muscle located side-by-side in the larynx (voicebox), between the trachea (windpipe) and the upper respiratory region (nasal passages, mouth, sinuses, and throat). When the vocal cords are partially or fully closed and air is exhaled from the lungs, the exhaled air causes the vocal cords to vibrate, generating sounds of various pitches and intensities. If the ability to open and close one or both vocal cords is impaired, the quality of a person's voice is diminished. Paralysis of one or both vocal cords may lead to a significant impairment or complete loss of the ability to speak.
Another potential source of diminished voice quality may be effects of a mechanical or electrical apparatus or attachment thereto located in proximity to a person's larynx and performing repeated actions with a certain frequency (e.g., about 50 Hz). At such a frequency, the vocal cords may resonate contrary to the volition of the person and diminish voice quality.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.